This application claims the priority benefit of Japanese application serial No. 2001-290447, filed on Sep. 25, 2001 and 2001-349198, filed on Nov. 14, 2001.
1. Field of the Invention
This invention relates in general to a charging device, which comprises a charging member, disposed opposite to a surface of an image supporter and pressed against the image supporter, wherein a charging voltage is applied to the charging member to discharge between the charging member and the surface of the image supporter, so as to charge the image supporter, and the charging member further comprises spacers in contact with a portion other than an image forming region of the image supporter, and a portion of the charging member opposite to the image forming region of the image supporter separates from the surface of the image supporter by a tiny gap. The invention also relates to an image forming unit with the image supporter and the charging member. The invention also relates to an image forming device having the above charging device.
2. Description of Related Art
Conventionally, it is well-known that the aforementioned charging device is used in an image forming device where an image supporter is charged by a charging device, the charged image supporter is then exposed to form an electrostatic latent image thereon, and the electrostatic latent image is visualized as a toner image. The image forming device can be an electronic copying machine, a facsimile, a printer, or a multi-function machine with at least two of the above functions. Since a portion of the charging member opposite to the image forming region of the image forming supporter of the charging device has a tiny gap rose from the surface of the image supporter, the drawback that the charging member is in contact with the surface of the surface supporter to contaminate the charging member can be suppressed, or the degradation of the surface of the image supporter at the early stage can be avoided.
If the tiny gap is too large, streamer discharge occurs when using this charging device; and therefore, the surface of the image supporter cannot be uniformly charged, so that a spotted abnormal image occurs on the toner image that is formed on the image supporter and the image quality degrades. Conventionally, the tiny gap between the charging member and the surface of the image supporter is set below 100 xcexcm to prevent the streamer discharge from occurring, so as to improve the image quality of the toner image. However, according to the study of the present invention, it can be understood that only setting the ting gap below 100 xcexcm has its limitation to improve the image quality of the toner image. The reason is further discussed as follows.
The charging device using the above charging member is used to discharge at the gap between the charging member and the surface of the image supporter so that the image supporter is charged. Discharge gas such as oxynitride is created by discharge, and the discharge gas is combined with the material in the air to form discharge products that will adhere on the surface of the image supporter. As the amount adhered to the surface increases, the discharge products absorb the water in the air and the resistance gets lower, so that the resistance of the surface of the image supporter is reduced. When the image supporter is charged, exposed to form the electrostatic latent image that will be visualized as the toner image, in general, the abnormal image such as the image stream or image fade occurs.
The abnormal image is highly related to the size of the tiny gap. It can be understood that when the tiny gap is set a certain suitable value below 100 xcexcm, the amount of the discharge products adhered onto the surface of the image supporter is minimized. As the tiny gap is larger, or in contrast, smaller than the optimum value, the amount of the discharge products adhered onto the surface of the image supporter increases. The explanation related to this point can be understood by the experiment example in the following description.
As can be realized from above description, if the tiny gap between the charging member and surface of the image supporter is set the optimum value or near that value, the amount of the discharge products adhered onto the surface of the image supporter is reduced, so that the occurrence of the abnormal image can be effectively suppressed, or can be avoided.
The charging member is pressed by a pressure means. Since the spacers of the charging member is pressed to contact with the image supporter, if the surface of the image supporter is slightly waved or slightly acentric, the pressing force applied to the charging member by the pressure means varies when the image supporter rotates. In addition, due to the impacting force applied to the image supporter, the image supporter in rotation vibrates, and therefore, the charging member jumps on the surface of the image supporter, so that the spacers is instantly separated from the surface of the image supporter by a little distance. Because the pressing force applied to the charging member varies or the charging member jumps over the image supporter, therefore even though the tiny gap is set to the optimum value while the image supporter stops, the tiny gap deviates from the optimum value greatly when the surface of the image supporter rotates to perform the charging operation. In this way, the amount of the discharge products adhered on the surface of the image supporter increases, and therefore, the occurrence of the abnormal image cannot be avoided.
According to the foregoing description, it is an object of the present invention to provide a charging device, wherein even though the pressing force applied to the charging member varies, the large variation of the tiny gap between the charging member and the surface of the image supporter can be stopped and therefore the occurrence of the abnormal image can be effectively suppressed.
The second object of the present invention is to provide an image forming unit with the above charging device, so that the image forming unit can effect the above advantages.
The third object of the present invention is to provide an image forming device with the above charging device, so that the image forming unit can effect the above advantages.
According to the objects mentioned above, the present invention provides a charging device, which comprises a charging member, disposed opposite to a surface of an image supporter and pressed against the image supporter, wherein a charging voltage is applied to the charging member to discharge between the charging member and the surface of the image supporter so as to charge the image supporter. The charging member further comprises spacers in contact with a portion other than an image forming region of the image supporter, and a portion of the charging member opposite to the image forming region of the image supporter separates from the surface of the image supporter by a tiny gap. The magnitude of a total load applied in perpendicular to the surface of the image supporter from the spacers is set 4 N to 25 N (Newton), and in a moving direction of the surface of the supporter, a contact width of a contact portion where the spacer is pressed to contact with the image supporter is set below 0.5 mm.
The magnitude of the total load is preferably set 6 N to 15 N. In addition, the tiny gap is set 20-50 xcexcm. The charging voltage where an AC voltage is overlapped to a DC voltage is applied to the charging member. In addition, the voltage between peaks of the AC voltage applied to the charging member is set more than two times of an initial charging voltage of the image supporter.
The surface of the charging member opposite to a discharge region is a curve that is gradually separated from the surface of the image supporter, from a nearest portion with respect to the surface of the image supporter to an upstream and a downstream sides in the moving direction of the surface of the image supporter, respectively.
The charging member is formed in a cylindrical shape, and the charging member is a rotatable roller. In addition, the tiny gap is set larger than a toner grain size of a toner image formed on the image supporter. The tiny gap is set larger than a grain size of a carrier in a developer used in a developing device that is to form the toner image on the surface of the image supporter.
The charging device can further comprise a cleaning member for cleaning up the surface of the charging member. The cleaning member is rotationally supported.
In the above charging device, the charging member further comprises: a conductive base body where the charging voltage is applied thereon; and a resistant layer fixed on the conductive base body. Protrusions are formed on a portion of the resistant layer other than the portion opposite to the image forming region of the image supporter, to protrude towards the surface of the image supporter, and the spacers are formed by the protrusions.
Alternatively, the charging member further comprises a conductive base body where the charging voltage is applied thereon; a resistant layer fixed on the conductive base body; and a surface layer, deposited on the resistant layer. The thickness of a surface portion where the surface layer is not opposite to the image forming region of the image supporter is thicker than that of a surface portion where the surface layer is opposite to the image forming region of the image supporter, and the spacers are formed by the thicker surface portion of the surface layer.
Alternatively, the charging member further comprises a conductive base body where the charging voltage is applied thereon; a resistant layer fixed on the conductive base body; and a surface layer, deposited on the resistant layer. The surface layer comprises a base material and an electron conductive agent. The volume resistance rate of the surface layer is set higher than that of the resistant layer.
The invention further provides a charging device, which comprises a charging member, disposed opposite to a surface of an image supporter and pressed against the image supporter, wherein a charging voltage is applied to the charging member to discharge between the charging member and the surface of the image supporter, so as to charge the image supporter. The charging member further comprises spacers in contact with a portion other than an image forming region of the image supporter, and a portion of the charging member opposite to the image forming region of the image supporter separates from the surface of the image supporter by a tiny gap. In a moving direction of the surface of the supporter, a contact width of a contact portion where the spacer is pressed to contact with the image supporter is set below 0.5 mm.
The charging voltage where an AC voltage is overlapped to a DC voltage is applied to the charging member. In addition, the voltage between peaks of the AC voltage applied to the charging member is set more than two times of an initial charging voltage of the image supporter.
The surface of the charging member opposite to a discharge region is a curve that is gradually separated from the surface of the image supporter, from a nearest portion with respect to the surface of the image supporter to an upstream and a downstream sides in the moving direction of the surface of the image supporter, respectively.
The charging member is formed in a cylindrical shape, and the charging member is a rotatable roller. Preferably, the tiny gap is set below 100 xcexcm. In addition, the tiny gap is set larger than a toner grain size of a toner image formed on the image supporter. The tiny gap is set larger than a grain size of a carrier in a developer used in a developing device that is to form the toner image on the surface of the image supporter.
The charging device can further comprises a cleaning member for cleaning up the surface of the charging member. The cleaning member is rotationally supported.
In the above charging device, the charging member further comprises: a conductive base body where the charging voltage is applied thereon; and a resistant layer fixed on the conductive base body. Protrusions are formed on a portion of the resistant layer other than the portion opposite to the image forming region of the image supporter, to protrude towards the surface of the image supporter, and the spacers are formed by the protrusions.
Alternatively, the charging member further comprises a conductive base body where the charging voltage is applied thereon; a resistant layer fixed on the conductive base body; and a surface layer, deposited on the resistant layer. The thickness of a surface portion where the surface layer is not opposite to the image forming region of the image supporter is thicker than that of a surface portion where the surface layer is opposite to the image forming region of the image supporter, and the spacers are formed by the thicker surface portion of the surface layer.
Alternatively, the charging member further comprises a conductive base body where the charging voltage is applied thereon; a resistant layer fixed on the conductive base body; and a surface layer, deposited on the resistant layer. The surface layer comprises a base material and an electron conductive agent. The volume resistance rate of the surface layer is set higher than that of the resistant layer.
The invention further provides an image forming unit, which comprises a charging member, as described above, and an image supporter. The image supporter and the charging member are integrally installed, and capable of detaching from or attaching to a main body of an image forming device. In addition, the image forming unit can further comprises a contact member that is in contact with the image supporter.
The invention further provides an image forming device, which comprises a charging device, equipped with a charging member as described above, and an image supporter. In the image forming device, the image supporter is formed as a photoreceptor having a surface layer made of amorphous silicon. Alternatively, the image supporter is formed as a photoreceptor having a surface layer where fillers are dispensed therein.